A recording density in an optical information recording/reproducing apparatus is inversely proportional to the square of diameter of a light spot formed on an optical recording medium by an optical head device. In other words, as the diameter of the light spot is smaller, the recording density is higher. The diameter of the light spot is inversely proportional to the numerical aperture of an objective lens of the optical head device. In other words, as the numerical aperture of the objective lens is higher, the diameter of the light spot is smaller. On the other hand, when the thickness of protective layer of the optical recording medium is deviated from a designed value (hereafter, such deviation is referred to as a protective-layer-thickness deviation), the spherical aberration caused by the protective-layer-thickness deviation distorts the shape of the light spot and deteriorates a record/reproduction characteristic. The spherical aberration is proportional to the fourth power of the numerical aperture of the objective lens. Thus, as the numerical aperture of the objective lens is higher, the safety margin of the protective-layer-thickness deviation of the optical recording medium for the record/reproduction characteristic is narrower. Hence, as for the optical head device and the optical information recording/reproducing apparatus in which the numerical aperture of the objective lens is increased in order to increase the recording density, the protective-layer-thickness deviation of the optical recording medium is required to be detected and compensated to prevent the deterioration of the record/reproduction characteristic.
As art for detecting and compensating the protective-layer-thickness deviation of the optical recording medium, Japanese Laid Open Patent Application (JP-P 2002-367197A) discloses optical head devices and an optical information recording/reproducing apparatus. Japanese Laid Open Patent Application (JP-P 2002-367197A) discloses two optical head devices. FIG. 1 is a schematic diagram showing a configuration of a first optical head device. With reference to FIG. 1, an emitted light from a semiconductor laser 101 is made parallel by a collimator lens 102, and about 50% thereof is transmitted through a beam splitter 123 and is condensed onto a disk 106 by an objective lens 105. The reflection light from the disk 106 is transmitted through the objective lens 105 in the opposite direction, and about 50% of the transmitted light is reflected by the beam splitter 123 and diffracted by a diffractive optical element 108d and received through a lens 109 and a cylindrical lens 124 by a photo detector 110c. 
FIG. 2 is a plan view of the diffractive optical element 108d from the incident direction of the light in the first optical head device. The diffractive optical element 108d includes diffractive gratings formed in a region 113i inside and a region 113j outside a circle. The diameter of the circle is smaller than the effective diameter (objective-lens-effective-diameter-inside region 160) of the objective lens 105 indicated by a dotted line in the figure. In the diffractive grating formed in the region 113i, the grating direction is parallel to the radial direction of the disk 106, and the grating pattern is straight lines of a constant interval. In the diffractive grating formed in the region 113j, the grating direction is parallel to the tangential direction of the disk 106, and the grating pattern is straight lines of a constant interval. About 40.5% of the incident light to each of the regions 113i and 113j is diffracted as ±1st order diffracted lights.
FIG. 3 shows a pattern of light receiving portions of the photo detector 110c, an arrangement of light spots on the photo detector 110c, and a configuration of a calculating circuit connected to the light receiving portions of the photo detector 110c in the first optical head device. Each of light receiving portions 114u and 114v is separated in four by two separation lines parallel to the radial direction and to the tangential direction of the disk 106. The two separation lines extend through the optical axis. The −1st order diffracted light from the region 113i of the diffractive optical element 108d is received by the light receiving portion 114v. The +1st order diffracted light from the region 113j of the diffractive optical element 108d is received by the light receiving portion 114u. 
The outputs from the light receiving portion 114v are connected to a differential amplifier 116b. The differential amplifier 116b calculates the difference between two outputs and two outputs to obtain a focus error signal for the inner portion of the light beam through an astigmatism method. The outputs from the light receiving portion 114u are inputted to a differential amplifier 116a. The differential amplifier 116a calculates the difference between two outputs and two outputs to obtain a focus error signal for the outer portion of the light beam through the astigmatism method. The outputs from the differential amplifier 116b and the differential amplifier 116a are inputted to an adder 125. The adder 125 calculates the sum of them to obtain a signal 117g. The signal 117g is the sum of the focus error signal for the inner portion of the light beam and the focus error signal for the outer portion of the light beam, and is used as a focus error signal that is used for a focus servo in order to compensate a focus deviation. The outputs from the differential amplifier 116b and the differential amplifier 116a are inputted to a subtracter 126. The subtracter 126 calculates the difference between them to obtain a signal 117h. The signal 117h is the difference between the focus error signal for the inner portion of the light beam and the focus error signal for the outer portion of the light beam, and is used as a protective-layer-thickness-deviation signal indicating the protective-layer-thickness deviation of the disk 106.
A second optical head device corresponds to the first optical head device in which the diffractive optical element 108d and the photo detector 110c are replaced with a diffractive optical element 108e and a photo detector 110d, respectively, and in which the cylindrical lens 124 is eliminated.
FIG. 4 is a plan view of the diffractive optical element 108e from the incident direction of the light in the second optical head device. The diffractive optical element 108e includes diffractive gratings formed inside and outside a circle having the diameter smaller than the effective diameter (objective-lens-effective-diameter-inside region 160) of the objective lens 105 indicated by a dotted line in the figure. The diffractive grating of inside is separated into two regions 113k and 131 by a straight line parallel to the radial direction of the disk 106. The straight line extends through the optical axis. The diffractive grating of outside is separated into two regions 113m and 113n by the straight line parallel to the radial direction of the disk 106. The straight line extends through the optical axis. In each region 113k, 113m, the angle of grating direction is positive with respect to the radial direction of the disk 106 and the grating pattern is straight lines of a constant interval. The radial direction extends through the optical axis. In each region 113l, 113n, the angle of grating direction is negative with respect to the radial direction of the disk 106 and the grating pattern is straight lines of a constant interval. The radial direction extends through the optical axis. About 40.5% of the incident light to each of the regions 113k, 113l, 113m and 113n is diffracted as ±1st order diffracted lights.
FIG. 5 shows a pattern of light receiving portions of the photo detector 110d, an arrangement of light spots on the photo detector 110d, and a configuration of a calculating circuit connected to the light receiving portions of the photo detector 110d in the second optical head device. Each of light receiving portions 114w and 114x is separated in four by two separation lines parallel to the radial direction and to the tangential direction of the disk 106. Each of light receiving portions 114y and 114z is separated by a separation line parallel to the radial direction of the disk 106. In the diffractive optical element 108e, the +1st order diffracted light from the region 113k and the +1st order diffracted light from the region 113m are received by the left side and the right side of the light receiving portion 114w, respectively, and the +1st order diffracted light from the region 113l and the +1st order diffracted light from the region 113n are received by the left side and the right side of the light receiving portion 114x, respectively. In the diffractive optical element 108e, the −1st order diffracted light from the region 113k and the −1st order diffracted light from the region 113m are received by the right side and the left side of the light receiving portion 114z, respectively, and the −1st order diffracted light from the region 113l and the −1st order diffracted light from the region 113n are received by the right side and the left side of the light receiving portion 114y, respectively.
The outputs from the light receiving portions 114y, 114z are inputted to the differential amplifier 116b. The differential amplifier 116b calculates the difference between two outputs and two outputs to obtain a signal 117j. The signal 117j is the sum of a focus error signal for the inner portion of the light beam and a focus error signal for the outer portion of the light beam, and is used as the focus error signal that is used for the focus servo. The focus error signals for the inner and outer portions are based on a knife edge method. The outputs from the light receiving portions 114w and 114x are inputted to the differential amplifier 116a. The differential amplifier 116a calculates the difference between four outputs and four outputs to obtain a signal 117i. The signal 117i is the difference between the focus error signal for the inner portion of the light beam and the focus error signal for the outer portion of the light beam, and is used as the protective-layer-thickness-deviation signal indicating the protective-layer-thickness deviation of the disk 106. The focus error signals for the inner and outer portions are based on the knife edge method.
As art for detecting protective-layer-thickness deviation in an optical recording medium, Japanese Laid Open Patent Application (JP-P 2000-155979A) discloses an optical head device and an optical information recording/reproducing apparatus. In the optical head device, a diffractive optical element separates the reflection light from the optical recording medium into the light of a ring band region and the light of the other region, and the former and the latter are received by different light receiving portions of a photo detector. The optical axis serves as a center of the ring band region. A focus error signal used for a focus servo is obtained from the output of the light receiving portion for receiving the latter light, and a protective-layer-thickness-deviation signal is obtained from the output of the light receiving portion for receiving the former light.
As art for detecting protective-layer-thickness deviation of an optical recording medium, Japanese Laid Open Patent Application (JP-P 2003-132582A) discloses an optical head device and an optical information recording/reproducing apparatus. In the optical head device, the reflection light from the optical recording medium is separated into two by a diffractive optical element or a beam splitter, a photo detector receives one by a light receiving portion arranged at the focal point and the other by a light receiving portion arranged at the position away from the focal point along the optical axis direction. Consequently, a focus error signal used for a focus servo is obtained from the output of the light receiving portion for receiving one light, and a protective-layer-thickness-deviation signal is obtained from the output of the light receiving portion for receiving the other light.
In the first optical head device and the optical information recording/reproducing apparatus of Japanese Laid Open Patent Application (JP-P 2002-367197A), the sum of the focus error signal for the inner portion of the light beam and the focus error signal for the outer portion of the light beam is used as the focus error signal used for the focus servo, and the difference between the focus error signal for the inner portion of the light beam and the focus error signal for the outer portion of the light beam is used as the protective-layer-thickness-deviation signal. Thus, in addition to the two differential amplifiers for obtaining the focus error signal for the inner portion of the light beam and the focus error signal for the outer portion of the light beam, the adder for obtaining the focus error signal used for the focus servo and the subtracter for obtaining the protective-layer-thickness-deviation signal are required, resulting in a complicated configuration of the electric circuit.
In the second optical head device and the optical information recording/reproducing apparatus of Japanese Laid Open Patent Application (JP-P 2002-367197A), in order to obtain the focus error signal used for the focus servo, the inner portion of the light beam and the outer portion of the light beam, which are separated by the diffractive optical element, are received by the same light receiving portion, and the size of the light receiving portion is large. Moreover, when the record or reproduction is performed on the optical recording medium of multilayer type, the reflection light from a layer other than a layer on which the record or reproduction is performed incidents on the light receiving portion as stray light. Thus, the record/reproduction characteristic is deteriorated. In this case, the light receiving portion of large size increases the amount of stray light which incidents on the light receiving portion and the record/reproduction characteristic is largely deteriorates.
In the optical head device and the optical information recording/reproducing apparatus of Japanese Laid Open Patent Application (JP-P 2000-155979A), in order to obtain the focus error signal used for the focus servo, only the light that includes the partial region of the reflection light from the optical recording medium is used. However, when the protective-layer-thickness deviation exists in the optical recording medium, the focus error signal obtained by using only the light that includes the partial region of the reflection light from the optical recording medium disagrees with the focus error signal obtained by using the light that includes the entire region of the reflection light from the optical recording medium in zero cross point, and does not indicate the correct focus deviation.
In the optical head device and the optical information recording/reproducing apparatus of Japanese Laid Open Patent Application (JP-P 2003-132582A), the protective-layer-thickness-deviation signal is obtained based on the phenomena that the protective-layer-thickness deviation of the optical recording medium causes an asymmetrical change in the focus error signal with respect to the direction of the focus deviation. However, since the asymmetrical change in the focus error signal caused by the protective-layer-thickness deviation of the optical recording medium is extremely small, it is difficult to obtain the protective-layer-thickness-deviation signal of high sensitivity by using the asymmetrical change.
As related art, Japanese Laid Open Patent Application (JP-P 2002-245639A) discloses an optical disk apparatus. The optical disk apparatus records information on and reproduces information from an optical disk in which a recording layer is formed above a transparent substrate. The optical disk apparatus condenses a light for record or reproduction of information through the transparent substrate of the optical disk on the recording layer. The optical disk apparatus includes: a signal detecting means for detecting a focus error signal and a focus sum signal from the return light reflected by the recording layer; and a thickness error detecting means for detecting a thickness error in the transparent substrate with respect to a specified value based on the property of the focus error signal.
Japanese Laid Open Patent Application (JP-P 2003-45047A) discloses an optical pickup apparatus. The optical pickup apparatus includes: an emitting optical system for condensing a light beam through an optically transparent layer of an optical recording medium onto a recording surface to form a spot; and a light detecting optical system for condensing the return light reflected from the spot onto a photo detector, and detects a focal point error of the light beam. The optical pickup apparatus includes a diffractive optical element which is arranged on the optical axis of the return light in the light detecting optical system and includes a ring band. The ring band extracts a circular ray component around a predetermined radius on a pupil from the return light. The predetermined radius corresponds to a maximum value in the wave surface aberration distribution on an exit pupil of the emitting optical system. The photo detector includes a spot light receiving portion for receiving the extracted ray component which is transmitted through the ring band, and a focal point error detecting circuit which is connected to the spot light receiving portion and detects the focal point error of the light beam based on a photoelectric conversion output from the spot light receiving portion.
Japanese Laid Open Patent Application (JP-P 2003-91856A) discloses an optical pickup. The optical pickup includes: a first light source and a second light source which emit light beams of different wave lengths, respectively; a means for synthesizing optical paths of the light beams emitted from the first light source and the second light source along the substantially same axes; a converging means for condensing the light beams on a signal recording surface of a recording medium; and a light receiving element for receiving the return lights as the light beams reflected by the signal recording surface of the recording medium. An optical element, which makes the return lights corresponding to the light beams emitted from the first light source and the second light source to incident onto the substantially same positions on the light receiving element, is arranged between the converging means and the light receiving element.